350 Journal of Comparative Neurology. 



(4) It is seen that the average numbers of fibers per gram 

 found for the three localities of the nerve respectively are con- 

 siderably different in the two groups, yet a comparison of the 

 proportional numbers of the one group with the proportional 

 numbers of the other group shows quite a degree of similarity. 

 This similarity in the proportion of ventral to dorsal root fibers 

 and distal fibers indicates a tendency to maintain a certain rela- 

 tion between the numbers during the growth of the animal. 



The figures of Table IV, representing only the condition 

 of affairs in the 6th nerve, may be utilized to obtain the rela- 

 tions to be found when all the spinal nerves are taken into 

 account. 



The figures of Birge, above referred to, including all the 

 spinal nerves also show that the smaller specimen possesses a 

 greater number of fibers in proportion to its weight than a 

 larger one. This, taken together with the fact that the smaller 

 specimen gains nerve fibers more rapidly than the larger one, 

 no doubt indicates that the subsequent increase in the size of 

 the animal does not consist so much in the formation of new 

 tissue elements to be innervated as it does in the increase in the 

 size of those already formed and innervated. However, newly 

 formed tissue elements may not be the only destination of the 

 added nerve fibers. It is highly probable that new nerve fibers 

 may go to tissue elements already having a nerve supply. Sin- 

 gle amphibian muscle fibers have been observed having from 

 two to six nerve terminations on them (Sandmann ('85) ). 

 Areas supplied by sensory fibers chiefly may also acquire new 

 fibers as the area grows in extent. 



Table V is simply an addition to Table IV and is intended 

 to give the rate at which fibers are gained as the frog gains in 

 weight. The table explains itself and shows that a frog 

 growing from 7. i grams to 60.9 grams (averages from Table 

 IV) must gain 72.8 ventral root fibers, 128. i dorsal root fibers, 

 and 282 fibers in the trunk and dorsal branches of its 6th. spinal 

 nerve. If we divide these figures by 53.8, the difference be- 

 tween 7, 1 and 60. 9, or the number of grams gained, we would 

 get for the 6th. nerve of one side, 1.35 ventral root fibers, 2.38 



